1. Field of the Invention
The present invention relates to a magneto-optical optical component used for an optical communication system, such as a variable light attenuator, a light modulator, or an optical switch.
2. Description of the Related Art
As one of magneto-optical components used for an optical communication system, there is a variable light attenuator. As the variable light attenuator, there is known a so-call magneto-optical variable light attenuator for controlling the amount of attenuation of light by changing a Faraday rotation angle according to the intensity of an applied magnetic field. Since the magneto-optical variable light attenuator comprises no mechanical movable part, there are merits that its reliability is high and miniaturization is easy. The magneto-optical variable light attenuator comprises a magneto-optical element (magneto-optical crystal) and an electromagnet for applying a magnetic field to the magneto-optical element. The intensity of magnetization of the magneto-optical element is changed by changing the amount of current flowing through a coil of the electromagnet to control the intensity of the magnetic field applied to the magneto-optical element, and the Faraday rotation angle can be controlled.
A method of controlling the magnetic field applied to the magneto-optical element is disclosed in, for example, patent document 1 (Japanese Patent No. 2815509). The magnetic field control method will be described with reference to FIGS. 22A and 22B. FIG. 22A shows a variable light attenuator, and the variable light attenuator comprises a Faraday rotator (magneto-optical element) 113 and a polarizer 112. Besides, the variable light attenuator comprises a permanent magnet 114 and an electromagnet 115 for applying magnetic fields to the Faraday rotator 113 in directions orthogonal to each other, and a variable current source 116 for supplying a driving current to the electromagnet 115.
The direction of the magnetic field applied to the Faraday rotator 113 by the permanent magnet 114 is parallel to the transmission direction of a light beam 117 in the Faraday rotator 113, and the direction of the magnetic field applied to the Faraday rotator 113 by the electromagnet 115 is perpendicular to the magnetic field application direction by the permanent magnet 114 in the Faraday rotator 113 and the transmission direction of the light beam 117.
In FIG. 22B, each of arrows 102 and 105 denotes a vector indicating the direction of magnetization in the Faraday rotator 113 and its magnitude, and each of arrows 101, 104 and 103 denotes a vector indicating the direction and magnitude of an applied magnetic field applied from outside. In the drawing, a Z direction denotes the direction of propagation of light in the Faraday rotator 113, and an X direction is orthogonal to the Z direction. The Faraday rotator 113 is brought into a state of the saturation magnetization 102 by the vertical magnetic field 101 generated by the external permanent magnet 114. Next, when the horizontal magnetic field 103 generated by the electromagnet 115 is applied, the external magnetic field becomes the combined magnetic field 104, and the Faraday rotator 113 is brought into a state of the magnetization 105. The magnitude of the magnetization 105 is equal to the magnitude of the saturation magnetization 102. Accordingly, the Faraday rotator 113 is in the state of the saturation magnetization.
As stated above, the Faraday rotator 113 is put in the state of the saturation magnetization by previously applying the vertical magnetic field to the Faraday rotator 113 by the permanent magnet 114, and the horizontal magnetic field is applied by the electromagnet 115 disposed in the in-plane direction of the Faraday rotator 113. The direction of the magnetization of the Faraday rotator 113 is rotated by the combined magnetic field 104 of the two magnetic fields from the magnetization 102 to the magnetization 105 by an angle of θ, and the magnitude of a magnetization component 106 in the Z direction is controlled. The Faraday rotation angle is changed dependently on the magnitude of the magnetization component 106. In the case of this method, since the Faraday rotator 113 is always used in the saturation magnetization region, hysteresis does not occur, and there is a feature that the Faraday rotation angle can be changed with high reproducibility.
However, in the magnetic field application method disclosed in the patent document 1, since the magnetization is uniformly rotated in the state where the magnetic field in the vertical direction is applied by the permanent magnet 114, it is necessary to increase the magnetic field in the in-plane direction applied by the electromagnet 115. Thus, it is necessary to use the large electromagnet 115 or to supply a large current to the coil of the electromagnet 115. Accordingly, there arises a problem that it is difficult to miniaturize the magneto-optical component and to reduce electric power consumption. Besides, since the miniaturization of the magneto-optical component is difficult, there arises a problem that it is very difficult to form an array in which a plurality of magneto-optical components are arranged.